1. Field of the Invention
The invention relates to a polyimide film having a polyimide as the primary constituent and containing at least a polymeric electroconductive material, an image-forming apparatus having an intermediate transfer belt made of the polyimide film, a method for producing the polyimide film, and a method for producing the intermediate transfer belt.
2. Description of the Related Art
Some image-forming apparatuses by electrophotographic process employ an endless intermediate transfer belt (see, e.g., Japanese Patent Application Laid-Open (JP-A) No. 62-206567). In the image-forming apparatuses equipped with an intermediate transfer belt, an image is formed by forming an electrostatic latent image on the surface of an image carrier, developing the electrostatic latent image with a toner into a toner image, transferring the toner image on the image carrier once onto the intermediate transfer belt electrostatically (primary transfer) by applying a bias voltage to the intermediate transfer belt circulating while in contact with the image carrier, finally retransferring the toner image on the intermediate transfer belt onto a recording paper and fixing the image thereon into a fixed toner image (secondary transfer).
The intermediate transfer belt is an endless element prepared by cutting a film-shaped base material into a long belt and connecting both ends of the belt to each other. Materials for use as the intermediate transfer belts hitherto proposed include conductive belts made of thermoplastic resin including a polycarbonate resin (see JP-A No. 6-95521); polyvinylidene fluoride (PVDF) (see JP-A Nos. 5-200904 and 6-228335); polyalkylene phthalate (see JP-A No. 6-149081), and blends of polycarbonate (PC)/polyalkylene terephthalate (PAT) (see JP-A No. 6-149083), ethylene tetrafluoroethylene copolymer (ETFE)/PC, ETFE/PAT, and PC/PAT (see JP-A No. 6-149079); and the like.
However, a conductive material made of a thermoplastic resin such as a polycarbonate resin, polyvinylidene fluoride (PVDF) or the like is inferior in mechanical properties, leading to larger deformation of the belt by the stress during operation and consequently to less reliable formation of high-quality transferred images. In addition, it also has a shortened belt life, due to cracks generated at the belt edges during operation.
Alternatively, intermediate transfer belts made of a thermosetting polyimide resin, which is superior in mechanical properties and heat resistance, and conductive filler dispersed therein is proposed (see JP-A Nos. 5-77252 and 10-63115). An intermediate transfer belt made of a carbon black-dispersed thermosetting polyimide was also proposed (see JP-A No. 63-311263).
However, such intermediate transfer belts carry a problem that the intermediate transfer belts made of a thermosetting polyimide resin containing a conductive filler or carbon black as the conductive material exhibit decrease in volume or surface resistivity by repeated application of bias voltages during the primary transfer, and thus could not provide high-quality transfer images reliably. They also carry a disadvantage that if a conductive filler or carbon black is dispersed in the polyimide resin, the resulting belt becomes brittle and more vulnerable to cracks.
As a new measure to overcome these disadvantages, an intermediate transfer belt having a polyimide as the primary constituent and containing at least polyaniline and a dopant for making the polyaniline conductive was proposed (see JP-A No. 2001-109277).